estar más tieso que la mojama 10
The formula "[verb] rather than [exaggerated situation]" is very common in proverbs. And in this case, perhaps it sounds better to "[be] stiffer than a mojama", and you could even change 'stiff' to 'hard', which is more associated with the concept of 'being without money'. But it can also be used to describe someone who is intoxicated with cocaine, or who is dead. Mojama or almojama ("air-cured tuna meat") has a stiff appearance once dried or jerky, and that's where the comparison comes from.